Extrinsic Curie temperature and spin reorientation changes in Nd{sub 2}Fe{sub 14}B/{alpha}-Fe nanocomposites

Abstract

Abstract:The Curie temperatures and spin reorientation temperatures of a series of four melt-spun nanocomposite materials comprised of Nd2FelB and varying amounts of a-Fe were measured using independent techniques. The phase constitution and grain size was assessed with synchrotron x-ray diffraction; the Curie temperatures were measured by differential thermal analysis (DTA) and & SQUID magnetometry in the temperature range 375 K s T s 800 K, whereas the spin reorientation transition temperature was determined from ac susceptibility measurements taken in the range 10 K s T I 300 K. The Curie temperature increases with increasing excess iron content, resulting in a 18" enhancement over the Curie temperature of pure Nd2Felfi for 27 wt% excess a-Fe. The spin reorientation temperatures are depressed from the singlecrystal value by an average of 10 degrees. Both anomalous effects are attributed to intergranular exchange coupling present in the alloys, although the effects of uncompensated stress between the constituent phases cannot be ruled out. The experimental results suggest that while the Curie temperature of the Nd2FelB phase may be extrinsically enhanced significantly beyond the bulk value, possibly extending the range of applications of this compound, the anisotropy may be simultaneously lowered, impeding the attainment of high coercivities in these alloys.